Luna/Counters

Bad against...

Bristleback

Quill Spray and Bristleback harass Luna more efficiently than she does with Lucent Beam.
Bristleback withstands Luna’s nukes, and gives lots of damage in return.
Viscous Nasal Goo and Warpath are enough to outrun a speedy Luna.
Bristleback tends to be a frequent carrier of Crimson Guard, which is good protection from bouncing glaives.

Broodmother

Her Spiderlings can harass Luna in lane heavily, as Luna has no AoE abilities to clear Spiderlings.
Broodmother can knock down towers in the early game quickly, and limit Luna's farming space.

Juggernaut

Because of her low range, Luna cannot easily destroy Healing Ward.
Both Blade Fury and Omnislash are direct counters to Eclipse, offering Juggernaut spell immunity.

Lina

Lina has very high magic burst damage to use against the low-health-pool Luna. Laguna Blade can also be upgraded with Aghanim's Scepter to bypass Luna's Black King Bar, making Lina even more of a threat.

Meepo

Meepo can tank Eclipse because of his clones.
Earthbind takes away Luna's mobility advantage.
Meepo can shutdown Luna's split-push ability with his global presence.

Phantom Lancer

Phantom Lancer creates many illusions to tank Luna's Eclipse.
Doppelganger can be used to dodge a number of Lucent Beams during Eclipse.
Phantom Lancer can chase Luna effectively regardless of her high movement speed thanks to Phantom Rush and Spirit Lance.
Although her Moon Glaives kill his illusions quickly, Luna's low health pool is easily exploited by the sheer damage output from his illusions.

Pudge

Flesh Heap's magic resistance allows him to survive Eclipse, and Meat Hook and Dismember can disable Luna through her Black King Bar.
However, Lucent Beam can still cancel Dismember from a safe distance, even if its damage is going to be reduced.

Sniper

Sniper usually sits at the back and it's hard for Luna to get close to him to attack.
Due to Sniper's Shrapnel and his long range, Luna has a hard time taking high ground despite her Moon Glaives being fantastic for taking Barracks and buildings.

Slark

Shadow Dance prevents Luna from damaging Slark, and together with Shadow Blade he can kite her or escape.
Luna has no way to escape from Pounce's leash.

Techies

Luna has low HP, low range and often stalks through jungle. She cannot properly disarm Proximity Mines and could fall victim to their traps.
Of course, if Luna happens to catch Techies off-guard, they won't be able to survive for long against Eclipse, Lucent Beam, and right-clicks.

Tinker

Tinker's March of the Machines and Laser both destroy Luna's small health pool and make it hard for Luna to initiate with Eclipse (and making it hard to land glaives in Laser's case).
Since Tinker typically buys Blink Dagger, Luna has a hard time catching him despite her high movement speed.
Tinker can easily disable and nuke Luna down with Dagon or Scythe of Vyse before she can use Lucent Beam or Eclipse.

Zeus

His highly damaging nuke spells with low cooldowns is not something Luna wants to go against.
Thundergod's Wrath can be used before a fight to harm Luna before she activates her Black King Bar, reducing her survivability for that fight.

Others

Other carries who can chase effectively despite Luna's high movement speed: Phantom Lancer, Slark, Chaos Knight, or Phantom Assassin.
Heroes that create illusions (such as Spectre, Shadow Demon, Phantom Lancer, Naga Siren, Chaos Knight, Terrorblade) or bring other units into the fight (such as Nature's Prophet, Lone Druid, Chen, Enchantress and the aforementioned Meepo and Arc Warden) can cause Eclipse to strike the illusions/summons unintentionally. (However, in many cases Moon Glaives can resolve this issue and eliminate them before activating Eclipse).
A couple of long-range heroes with excellent laning like Mirana, Lina, Necrophos, Death Prophet. The key to success is sticking together and attacking Luna whenever she enters the range, without even giving Luna a chance to retaliate. Multiple long-range heroes is a reason to pick another hero.
Every hero that can withstand or prevent Eclipse at level 7: Juggernaut, Riki, Clockwerk, Drow Ranger, Abaddon, Faceless Void, Weaver, Outworld Devourer.
A couple of nukers and disablers on the same lane can finish a squishy Luna efficiently despite a teammate's assistance: Sven + Shadow Shaman.
Escape-type carries: Slark, Storm Spirit, Weaver.
All assassin-type carries in a one against one situation: Meepo, Phantom Assassin, Riki, Ursa.

Items

Items which allow the enemy to go invisible, such as Glimmer Cape and Shadow Blade. Silver Edge will also disable Moon Glaives as an added effect.
Because of Luna's low HP and strong indiscriminate AoE damage, Luna suffers greatly from the Last Will of the Necronomicon warrior.
Vanguard/ Abyssal Blade/ Crimson Guard greatly reduce damage from bouncing glaives.

Good against...

Arc Warden

Luna's Moon Glaives allow her to clear Arc Warden's Tempest Double.
Luna's short range and fast movement speed allow her to fight Arc Warden inside his Magnetic Field.

Bane

It is almost impossible for Bane to use Fiend's Grip properly against Luna's allies due to the huge cast range of the Lucent Beam.

Bounty Hunter

Even though Luna is the kind of hero that Bounty Hunter can handle due to Shadow Walk rendering him safe from Eclipse, smart Luna will always buy Dust of Appearance, allowing her to provide vision on him to her team, and possibly, kill Bounty Hunter with Eclipse.

Chen

Moon Glaives can easily clear Chen's creeps while Eclipse can kill Chen's allies before he can react.

Enchantress

Enchantress has pitiful HP so Lucent Beam and Eclipse can kill her very quickly.
Moon Glaives allows Luna to clear Enchantress' creeps very efficiently.

Legion Commander

Provided Luna is able to cast it in time, Eclipse can help win a Duel from Legion Commander.
If Luna isn't the target, she can easily use Lucent Beam and Eclipse to win the fight for the ally.

Lifestealer

Although he can avoid Eclipse with Rage and Infest, Luna's physical damage can be enough to deal with him.
Manta Style is a common pick on Luna, which can help her dispel Open Wounds. In addition, Lifestealer is not good against illusions created by Manta Style.

Magnus

If Luna can use Eclipse in anticipation, Magnus will be killed before he can even manages to cast Reverse Polarity.
Aghanim's Scepter upgraded Eclipse is a very good counter-initiation tool against Magnus when he casts Reverse Polarity, granted as long as Luna can cast it from safe distance.

Necrophos

Though Luna has a hard time laning against Necrophos, his Ghost Shroud won’t protect him in teamfights because it will increase the damage from Lucent Beam and Eclipse.

Pugna

Life Drain will be always canceled by Lucent Beam if Pugna does not have Black King Bar.
Decrepify can be suicide against Luna due to Lucent Beam and Eclipse.
Nether Ward is rather weak against Luna since she does not have abilities that cost a lot of mana.

Riki

Luna can push very fast with Moon Glaives, which is something Riki cannot deal with well.
Luna is naturally very fast, making it hard for Riki to trap her in Smoke Cloud.
Even though Riki can evade Eclipse with his invisibility, smart Luna will always buy Dust of Appearance, providing vision on Riki to her allies and herself.

Tidehunter

Luna often builds Black King Bar, and none of Tidehunter's spells can bypass spell immunity.
Lucent Beam and Eclipse deal a lot of magic damage in the early and mid-game, which can bypass Kraken Shell.

Underlord

Lunar Blessing gives allies bonus damage and armor required to endure against Atrophy Aura early on.
Eclipse can be devastating for Underlord's team when they're grouped up for Dark Rift, it can also be used to intercept arrivals with Dark Rift. And Pit of Malice does not prevent Luna to use it.
To make things even better, Aghanim's Scepter upgraded Eclipse can finish off Underlord or any of his teammates escape attempts with Dark Rift if timed correctly.

Winter Wyvern

Cold Embrace is no salvation against Lucent Beam and Eclipse.
Moon Glaives will not hit a hero affected by Winter's Curse, making it weak against allies, but strong versus enemies.
Luna has better night vision provided by Lunar Blessing than Winter Wyvern when affected by Arctic Burn.

Others

Though Luna isn’t a dedicated chaser, she’s rather good at finishing off low HP heroes thanks to her high movement speed and incredible range on Lucent Beam.
Luna has a love/hate relationship with illusions and summons. On one hand, they can tank damage from Eclipse, but on the other hand, she can easily clear them before Eclipse with Moon Glaives.
Luna's Eclipse can be used against gap-closing heroes who use their blinking abilities without protection or who recklessly jump on her, provided that Luna is able to cast it in time.
With Dust of Appearance, Luna will be able to use Eclipse on invisible heroes. Be mindful that Slark is exception because he'll be able to remove the debuff with Dark Pact.
Lucent Beam can be used to cancel channeling abilities or abilities with long cast animations, and can be also used to prevent the escape attempts Town Portal Scroll and/or Boots of Travel.

Items

Black King Bar and other forms of Spell Immunity, though they will block Luna's magical damage output, they will not be mitigated by Luna's high physical damage.
Many carries have a hard time against heroes that buy Ghost Scepter and Ethereal Blade. However, items and abilities that turn the user ethereal are useless against Luna because Eclipse will deal enormous damage, especially when Luna farms Aghanim's Scepter, and she often uses her magical damage in order to kill heroes.
Illusions created from Manta Style are normally fragile, and Luna will easily destroy them. Avoid building Manta Style against Luna unless you have natural illusions ( Mirror Image etc) or illusion-related abilities (e.g. Mana Break).
Luna can put Linken's Sphere on cooldown using Lucent Beam, provided that she has -4s talent for Lucent Beam.
Luna's Eclipse is useful for intercepting heroes who teleport with Town Portal Scroll and/or Boots of Travel. She can also use Lucent Beam to prevent the escape attempts with it.

Works well with...

Crystal Maiden

Arcane Aura allows Luna to spam Lucent Beam more frequently.
Frostbite and Crystal Nova helps to chase heroes.

Keeper of the Light

Chakra Magic can sustain Luna's mana pool and give her unexpectedly very low cooldown (up to 0 seconds) nuke, especially in early game. Heal from Illuminate is very useful as well.

Night Stalker

Eclipse causes day to become night, triggering Night Stalker's Hunter in the Night for the ten extra seconds of nighttime.
Dark Ascension gives both of them long vision range.

Vengeful Spirit

Vengeance Aura can greatly improve Luna's attack damage in the late game. Early-game, she can set up kills as well as use Nether Swap on Luna to allow her escape from a sticky situation and potentially save her life.